High performance lithium-ion secondary batteries that exhibit high energy density, fast charge/discharge cycles and long cycle life have become increasingly important for the rapid development of the hybrid and electric vehicle industry. In addition, large format lithium-ion batteries look set to also play an important role in energy storage for renewable and off-peak electricity generation.
Lithium ion secondary batteries typically comprise two electrodes (a cathode and an anode) having a porous separator and liquid electrolyte material positioned between the two electrodes. At least one of the electrodes, typically the cathode, comprises a metal substrate (which acts as a current collector) and an electrode material applied by coating to the metal substrate. The cathode electrode material typically comprises a mixture of a lithium-containing compound that provides a lithium ion source in the battery, a binder, a solvent and conductive particulate material. The anode material typically comprises a carbon or graphitic type compound that intercalates lithium and a binder, a solvent and conductive particulate material. Aluminium metal is usually the substrate for the cathode material. Copper metal is usually the substrate for the anode material.
The lithium containing compound in the electrode material may be, for example, a lithium iron phosphate material (LFP), LiMXO4 (M: Fe, Mn, Co, Ni, etc and mixtures of these elements; X: P, Si, Si, V, etc and mixtures of these elements), Li2FePO4F, LiCoO2, LiCo1/3Ni1/3Mn1/3O2, LiMn2O4, Li2MnO3—LiMO2 (M: Mn, Ni, Co), Li2SO4, lithium vanadium oxides, lithium vanadium phosphates, lithium titanates, other lithium ion intercalating compounds and/or mixtures and composites of any of the aforementioned materials. The lithium containing compound in the electrode material can provide lithium ions to the electrolyte and receive lithium ions from the electrolyte during charging and discharging cycles.
Although the lithium containing compounds allow for higher electrical energy density during discharge and also allow for a large number of charging cycles, they suffer from the drawback that they have relatively low electrical conductivity. To overcome this difficulty, the electrode material is typically provided with conductive particulate material, such as electrically conductive carbon black. The carbon black provides increased electrical conductivity to the coating. In other electrode coatings, the lithium containing compound (which is typically in the form of solid particles) may be coated with a graphite layer to provide increased electrical conductivity.
Manufacturing of electrodes for lithium batteries typically involves coating the metal substrate with a layer of the electrode material. Manufacturing techniques have developed to rapidly apply a coating of the electrode material. These techniques require that the coating include a solvent. A solvent that is commonly used is N-Methyl-2-pyrrolidone (NMP). A binder, which is normally poly vinylidene fluoride (PVDF), is also included.
In order to form the electrode, it is common to provide the electrode material in the form of an ink or a liquid composition that can be applied to the metal substrate using technology similar to printing technology. After the coating of the electrode material has been applied to the substrate, the coating is subjected to a calendaring process to press and heat the coating, which increases the density of the coating.
Two aspects central to achieving high performance in these batteries is good internal ionic and electrical conductivity. In the case of electronic conductivity it is essential to have low impedance at the interface between the active cathode material and the metal electrode substrate. This is achieved by ensuring complete adhesion of the cathode to the metal substrate.
Adhesion between a coating and a substrate is usually achieved through one or more of the three following mechanisms:                Surface roughness;        Chemical bonding; and/or        Interface reaction or compound.        
With surface roughness, although the compounds that make up the substrate and the coating do not necessarily establish a chemical bond, the mechanical interlocking of their rough interface guarantees a good contact between the coating and the substrate.
With chemical bonding, the coating must contain ingredients or components that are capable of establishing molecular bonds to the substrate. This would ideally be the case, for instance, with most binders that are used in the battery cathode ink formulation.
Through an interface reaction, one (or more) new compound(s) is typically formed at the interface and this compound is likely to acquire the features that facilitate either one or both of the previous two mechanisms of adhesion—surface roughness and chemical bonding.
In the case of lithium-ion secondary batteries significant problems have arisen in achieving good adhesion at this interface between the electrode material and the metal substrate. Coated electrodes of this type need to maintain their conductivity while still being flexible enough to be wound or rolled into final battery shapes. As a result, lithium containing cathode materials are mixed with binders, conducting carbon particles and solvent to produce an ink that can be cast onto a continuous roll of metal foil, such as aluminium, that can be later cut into appropriate lengths and wound with other components to produce the battery. Furthermore, when good adhesion is achieved between the cathode and the collector substrate, it is possible to exert higher pressures during the process step known as calendering. Such additional pressure will in turn improve the electrical contact between particles, and also, enhance the packing density, both of which are desirable for enhanced performance. FIG. 1 shows the gradual reduction in resistance of pressed pellets of plain LFP powder without additional particulate Super P (Super P is the trade name of a carbon black material commonly used in lithium battery manufacture) or binder. As the contact between particles increases, so does the conductivity.
As mentioned above, a typical cathode ink composition may contain a lithium metal oxide, as the lithium-ion source, polyvinylidene fluoride (PVDF) or PVDF copolymer resins as the binder, N-Methyl-2-pyrrolidone (NMP) as the binder solvent and carbon black as a conductive particle source. Factors such as surface charges, reactions between components and final ink pH can result in a mix that has little or no adhesion with metal electrode substrates. In particular, a good mixing and distribution of the particulate conducting carbon needs to be obtained in order to ensure good electrical contact across the entire area and thickness of the cathode coating.
Aluminium, which is usually the substrate for the cathode material, is known to have a nanometer scale oxide surface layer. Caustic solutions and very acid solutions dissolve the oxide layer, however, the speed of re-oxidation after exposure in air at room temperature is known to be very fast, of the order of microseconds, for the re-establishment of the first few nanometers of the aluminium oxide coating. Such oxide layer is expected to change the surface properties of the substrate and to provide some degree of electrical insulating contribution.
Cleaning the aluminium substrate with caustic and acidic solution to remove the aluminium oxide layer from the surface of the substrate is possible. However, it is unlikely that such a step will be a viable processing step in electrode manufacture, not only because it introduces additional steps, but primarily because the aforementioned oxide layers will re-form quickly, prior to the final coating step with the electrode material. In addition, KOH is known to react exothermically and somewhat violently with aluminium, with generation of hydrogen gas. Thus, it would be difficult to implement on a large scale.
Finally in order to achieve good adhesion, in addition to introducing one of the aforementioned adhesion mechanisms, a good mixing and distribution of the particulate conductive material, such as conducting carbon, needs to be obtained in order to ensure good electrical contact across the entire area and thickness of the coating.
A number of approaches have been used in an attempt to overcome this adhesion problem. These approaches include pre-coating the metal electrode with an interface layer, pretreating the metal surface using pickling solutions to enhance surface roughness and adding functional groups that enhance cross linking of monomers with PVDF copolymers binders.
A number of efforts, from prior art, to improve the coating of electrode materials on to metal substrates for use in production of batteries are listed below.
United States patent application US 2009/0263718A1 discloses the addition of two different size ranges of particulate conductive carbon (one with average particle diameter of from 3 to 10 micron and another with average particle diameter of 1 micron or less) is useful to improve the cohesiveness in the pressing stage and assists to prevent defects such as detachment from occurring.
United States patent application US 2009/0155689A1 discloses the use of multimodal particle size distribution in the LiMPO4 (M: Fe or Mn) active cathode material comprising at least one fraction of micron size particles and at least one fraction of submicron size particles in order to enhance packing density and optimise porosity. Two different processes are in general used to obtain the materials with different particle size distribution. Although the focus is to enhance energy density and power performance, improvements in cohesiveness similar to those of patent application US 2009/02673718A1 may be expected.
United States patent application US 2004/0234858A1 discloses that when a surface roughness of at least 0.1 micron in the current collector is used, adhesion between the mixed layer and the collector is greatly improved.
U.S. Pat. No. 5,399,447 discloses a method to reduce the acidity of an adhesion promoter layer made of carbon and polyacrylic acid by treating this layer with LiOH. Otherwise, there is the risk of H+ ions taking the place of Li+ ions in the cathode material, thereby reducing the capacity of the battery.
International Patent application WO 00/49103 describes a method for the adhesion of vinylidene fluoride resins to metal substrates which is characterized in that, when sticking a polyvinylidene fluoride to a metal substrate, there are added to and mixed with vinylidene fluoride resin (a) at least one type of polymer (b) selected from acrylic and methacrylic polymers or resins containing such polymers and at least one organic compound (c) selected from the mercapto, thioether, carboxylic acid and carboxylic anhydride groups.
A method of manufacturing electrodes for electrochemical devices is disclosed in United States patent application US 2006/0153972A1. In this patent, adhesion is attributable to an electrically conductive adhesive produced by mixing a particulate rubber and particulate conducting carbon. The role of too little or too much rubber in proportion to carbon is emphasized.
Although the prior art mentioned above teaches ways to improve adhesion, most of these methods introduce additional processing steps in the manufacture of the battery cathodes adding to the complexity and cost. Most battery manufacturers are reluctant to alter their manufacturing practices significantly. Therefore, it is desirable to provide for enhanced adhesion of the electrode material to the metal substrate without requiring the introduction of additional steps to the electrode manufacturing process. Desirably, enhanced adhesion of the electrode material should be obtained without requiring any other changes in the coating manufacturing steps and without any collector/substrate pre-treatment in the lithium-ion secondary battery manufacturing industry, in particular, for powders with various particles characteristics and morphologies.
Throughout this specification, the term “comprising” and its grammatical equivalents shall be taken to have an inclusive meaning unless the context of use indicates otherwise.
The applicant does not concede that the prior art discussed in this specification forms part of the common general knowledge in Australia or elsewhere.